The new director of the United States' National Security Agency has acknowledged it uses facial-recognition tools but said the intent was primarily to identify terrorists and help prevent attacks.

"We do not do this on some unilateral basis against US citizens," said Admiral Michael Rogers, in some of his first public remarks since taking the helm of the embattled spy agency two months ago.

A year after the first leaks emerged about the scope of NSA surveillance programmes, Rogers is seeking to reframe the debate that has damaged the reputation and morale of the NSA. He says the public needs to understand not just what the organisation does but also why it does it and under what limits.

Rogers said the agency did not have access to drivers' licences or passport photo databases to search for Americans.

The New York Times has reported that the agency harvests about 55,000 "facial recognition quality images" on a daily basis as part of its global surveillance operations.

"Our mission at the NSA is very explicit - foreign intelligence and [cybersecurity]," Rogers said. "If we do anything against US persons, we have specific legal constraints that we must apply. We just don't unilaterally decide, 'Hey, today I'm going after Citizen X, Y and Z'."

Leaks by former NSA contractor Edward Snowden have revealed, for instance, that the agency has been collecting and recording an entire foreign nation's phone calls.

Rogers argued that the public had not focused on the protections afforded Americans in its collection efforts. "If we come to a realisation that someone we're tracking has a US connection that we were unaware of, in broad terms, we have to stop what we're doing," he said. "If we think there's a legal basis to this, then we have to get a legal authority or justification" to continue.

But he did not address what happens with the communications of Americans who are not targeted but whose data is swept up anyway, either because they are in contact with a target or because of the way data is collected. How much of that data is being collected today, how often that data is searched for Americans' communications, and how strong the protections ought to be are still matters of debate.

Rogers also sought to broaden public concern beyond what the spy agency is doing to the vast amounts of digital data being collected through other means or by private companies - "whether it's the cameras out on the street or every one of your personal digital devices constantly asking you, 'Can I share where you are?'"

He added: "We have framed this debate way too narrow… This is much bigger than the National Security Agency."

But Rogers did not discuss the deeper questions of why it is in the nation's interest for the agency to collect such large amounts of digital data and whether pervasive surveillance will diminish user trust in the internet.

Asked his opinion of Snowden, who was recently interviewed by NBC News, Rogers said he seemed to be "an intelligent individual, articulate". But, he added: "He seemed fairly arrogant… I fundamentally disagree with what he did."

Asked if he thought Snowden, who is in Russia under temporary asylum, could have been be a double agent, Rogers said: "Probably not." But he said an investigation continued.